The specific absorption rate SAR is a measure for the absorption of electromagnetic fields especially in biologic tissue. The specific absorption rate SAR is moreover an important indicator for the irradiation of a high-frequency transmission signal from a radio system, for example a handheld radio transceiver, and its coupling into the human body. The coupling of the high-frequency transmission signal into the human body represents a risk factor in the use of handheld radio transceivers, for example mobile phones, because the absorption of the electromagnetic field energy leads in any case to a warming of the tissue, which can lead to changes in the tissue, which can be the cause of cancer.
There is therefore the necessity to hold the transmission power of handheld radio transceivers, especially if they are situated near the human body, as low as possible, in order to reduce the level of contamination for the body. A statement concerning the coupling of the high-frequency transmission signals into the human body could be used in order to adapt the transmission power of the handheld radio transceiver accordingly.
Two different factors have a substantial influence on the specific absorption rate SAR of a handheld radio transceiver, i.e.    Factor 1: the transmission power effectively irradiated by the handheld radio transceiver (HF transmission power), and    Factor 2: the proximity of the antenna or antennas to the human body.
In order to be able to determine or estimate the specific absorption rate SAR as exactly as possible, it is advantageous to evaluate or to detect both factors. I.e., that advantageously both the HF transmission power effectively irradiated and the proximity of the antenna(s) to the body are considered in the determination or valuation of the specific absorption rate SAR. If both factors are considered, a good approximation of the power coupling to the real power coupling into the human body is possible.
If however only one of the two factors is considered, for the other factor the worst case has to be presumed in order to maintain the level of contamination for the body as low as possible. This however has the disadvantage that the transmission power has to be reduced beyond the necessary, which has a negative effect on the transmission properties of the radio system or handheld radio transceiver.
For the detection of the HF power effectively irradiated by an antenna of a handheld radio transceiver and for the detection of the distance of a human body from the handheld radio transceiver or from the antenna of the handheld radio transceiver, solutions are known from prior art. One solution for detecting the irradiated HF power is shown in regard to FIG. 1. FIG. 2 shows a solution for detecting the distance of a human body from an antenna of a handheld radio transceiver.
FIG. 1 shows an approach to a solution known from prior art for detecting the HF transmission power effectively irradiated by an antenna. A HF transmitter, which here consists of a signal generator for generating a HF-carrier signal, a signal production, by which the transmission information to be transmitted is provided, and a mixer, generates a modulated HF transmission signal which is led to a power amplifier PA. With the power amplifier PA the HF transmission signal is brought up to transmission level. The amplifier signal is then, by a circulator and an antenna switch, led to the antenna A, where it is finally irradiated.
The circulator itself is not necessary for normal operation. The circulator is provided here in order to allow in the normal operation a SAR valuation of the signal irradiated at the antenna.
By the circulator the waves reflected by the antenna A are diverted to a demodulator, so that the reflected waves do not appear on the power amplifier PA. The power amplifier PA is always well adapted and delivers its rated power, so that the rated power can be supposed to be constant. In this way it is possible, exclusively based on the changing power of the reflected wave to make a statement concerning the power effectively irradiated by the antenna A. The demodulator here fulfills the function of a rectifier, because only a measure for the power is needed, but not for the transmission information itself. The demodulator is coupled with a SAR evaluation unit and can consist of a DC voltage amplifier and an analog-to-digital converter or a simple comparator. The exact configuration of the SAR evaluation unit mainly depends on how precisely the SAR-valuation has to be.
The approach to a solution shown in FIG. 1 for detecting the HF transmission power effectively irradiated by the antenna is especially suitable for systems with constant envelope, like for example GMSK-/GFSK systems (GSM/DECT). With this approach to a solution above all the direct correlation of the SAR estimation with the HF transmission signal is advantageously exploited, because the power effectively irradiated is detected exactly where the transmission signal produces the coupling into the human body, i.e. at the antenna.
FIG. 2 shows an approach to a solution known from prior art for detecting an object or human body near the handheld radio transceiver. For detecting a distance or an approach of an object or human body to a handheld radio transceiver here a capacitive sensor is provided, which works according to a so-called loading method. In a capacitive sensor operating according to the loading method a sensor electrode is fed with an electric alternating signal of a generator G1. By means of a receiver E1 (or reception path E1) an evaluating device can detect and evaluate the capacitive load of the generated signal. The evaluator can be a component of a SAR evaluation unit.
Generally speaking, the smaller the distance of the electrode from the object or the human body, the greater is the detectable capacitive load. It is however disadvantageous that for a sufficiently good correlation between the antenna signal and the sensor result of the capacitive sensor the electrode must be close to the antenna. By the proximity of the sensor electrode to the antenna the adjustment of the antenna is reduced, so that worse transmission/reception properties for the handheld radio transceiver result. In order to not substantially influence the transmission/reception properties of the radio system by the sensor electrode, it is therefore necessary to arrange the sensor electrode at a certain distance from the antenna. In this way however a correlation between the antenna signal and the sensor result of the capacitive sensor cannot always be guaranteed. In order to nevertheless ensure that the level of contamination for the human body does not exceed a predetermined value, it is therefore necessary to reduce the transmission power of the handheld radio transceiver more than necessary.